This is a note to let you know that I've just added the patch titled
cgroup: avoid attaching a cgroup root to two different superblocks
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
cgroup-avoid-attaching-a-cgroup-root-to-two-different-superblocks.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From bfb0b80db5f9dca5ac0a5fd0edb765ee555e5a8e Mon Sep 17 00:00:00 2001
From: Zefan Li <lizefan@xxxxxxxxxx>
Date: Fri, 7 Apr 2017 16:51:55 +0800
Subject: cgroup: avoid attaching a cgroup root to two different superblocks
From: Zefan Li <lizefan@xxxxxxxxxx>
commit bfb0b80db5f9dca5ac0a5fd0edb765ee555e5a8e upstream.
Run this:
touch file0
for ((; ;))
{
mount -t cpuset xxx file0
}
And this concurrently:
touch file1
for ((; ;))
{
mount -t cpuset xxx file1
}
We'll trigger a warning like this:
------------[ cut here ]------------
WARNING: CPU: 1 PID: 4675 at lib/percpu-refcount.c:317 percpu_ref_kill_and_confirm+0x92/0xb0
percpu_ref_kill_and_confirm called more than once on css_release!
CPU: 1 PID: 4675 Comm: mount Not tainted 4.11.0-rc5+ #5
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
Call Trace:
dump_stack+0x63/0x84
__warn+0xd1/0xf0
warn_slowpath_fmt+0x5f/0x80
percpu_ref_kill_and_confirm+0x92/0xb0
cgroup_kill_sb+0x95/0xb0
deactivate_locked_super+0x43/0x70
deactivate_super+0x46/0x60
...
---[ end trace a79f61c2a2633700 ]---
Here's a race:
Thread A Thread B
cgroup1_mount()
# alloc a new cgroup root
cgroup_setup_root()
cgroup1_mount()
# no sb yet, returns NULL
kernfs_pin_sb()
# but succeeds in getting the refcnt,
# so re-use cgroup root
percpu_ref_tryget_live()
# alloc sb with cgroup root
cgroup_do_mount()
cgroup_kill_sb()
# alloc another sb with same root
cgroup_do_mount()
cgroup_kill_sb()
We end up using the same cgroup root for two different superblocks,
so percpu_ref_kill() will be called twice on the same root when the
two superblocks are destroyed.
We should fix to make sure the superblock pinning is really successful.
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/cgroup.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2105,7 +2105,7 @@ static struct dentry *cgroup_mount(struc
* path is super cold. Let's just sleep a bit and retry.
*/
pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
- if (IS_ERR(pinned_sb) ||
+ if (IS_ERR_OR_NULL(pinned_sb) ||
!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
mutex_unlock(&cgroup_mutex);
if (!IS_ERR_OR_NULL(pinned_sb))
Patches currently in stable-queue which might be from lizefan@xxxxxxxxxx are
queue-4.4/cgroup-avoid-attaching-a-cgroup-root-to-two-different-superblocks.patch